Hmm, these two statements seem to be in opposition to each other
What applications is it struggling with? What kind of performance are we talking - minutes for a task? Hours for a task?
Seems like you're a bit conflicted about whether you've got enough performance already or not. Have you been doing system monitoring to see just how often it's getting maxed out on a single core, how often it's ramping up into a higher clock state, and how much time it's just ticking along at a standard idle?
Pretty much any FM2 processor is going to have identical power draw at idle. It's literally just when you're loading the cores that you're going to see any difference at all. If it's spending 99% of its time at idle then a higher power processor is barely going to impact your power bill at all. But then, if you're at idle 99% of the time, you're not going to see much improvement from a faster processor because it'll only be getting used 1% of the time.
I think my inclination in your position would be to hold off until I was ready to build a new server. It doesn't sound like the performance is a real issue yet - just that you'd like it to be a bit faster. And as you've said yourself, you're limited in just how much extra single-threaded performance you can get out of the FM2 platform. Sit on it for now, bide your time, and stash that money away for a proper server rebuild when the time is right